(1) Field of the Invention
The present invention relates to a magnetic disk apparatus, a method for determining a data track pitch, and a self-servo write method.
(2) Description of the Prior Art
In recent years, magnetic disk apparatuses have been rapidly reduced in size and increased in capacity. For example, as for an increase in the capacity of magnetic disk apparatuses, the track density of disks is increasing, and track pitch is being further reduced. Therefore, in order to accurately record/reproduce data on/from a magnetic disk, it is necessary to precisely position a magnetic head at a target track among the tracks formed at a small pitch.
The position of a magnetic head is detected by allowing the magnetic head itself to read out servo signals recorded in advance on a magnetic disk at a certain angular spacing. The positioning of the magnetic head is controlled based on the servo signals. Specifically, from the detected magnetic head position information, a position error signal indicating the position error of the magnetic head with respect to the target track is generated, and the positioning of the magnetic head is controlled such that the magnitude of the position error signal is minimized. Accordingly, in order to position the magnetic head with high precision, servo signals serving as the reference for the positioning of the magnetic head must be precisely recorded on the magnetic disk.
Generally, methods for recording servo signals on a magnetic disk include: (1) a method for using a dedicated servo track recording device; (2) a method for performing magnetic printing method; and (3) a self-servo write method. Hereinafter, these methods will be briefly described.
According to the method for using a servo track recording device, in recording servo signals, while a magnetic head of a magnetic disk apparatus is precisely positioned by an external actuator provided in the servo track recording device, the servo signals are sequentially recorded using the magnetic head. However, the servo track recording device is expensive. Further, in order to record servo signals for a single magnetic disk apparatus, a period of time in the range of several tens of minutes to about one hour is required. Therefore, the expensive servo track recording device is exclusively used for a long period of time, and servo signals cannot be recorded on the other magnetic disk apparatus during that time. Thus, it is pointed out that the method for using a servo track recording device presents problems in terms of productivity and cost. Furthermore, since the magnetic head is positioned by the external actuator, a clean environment is required in order to avoid intrusion of dust into the magnetic disk apparatus.
According to the method for performing a magnetic printing method, a master disk, on which a pattern corresponding to servo signals is formed, is prepared, and the transfer recording of the servo signals is performed on a magnetic disk at once using the master disk. As such a method, for example, the following method is proposed: a master disk, on which a ferromagnetic thin film pattern corresponding to servo signals is formed, is brought into intimate contact with a magnetic disk, and an external magnetic field is applied, thus performing the transfer recording of the servo signals on the magnetic disk at once (see, for example, Japanese Patent No. 3323743). According to this method, the need for a servo track recording device is eliminated, and servo signals can be recorded with high precision for an extremely short period of time.
According to the self-servo write method, a reference signal, which is used as the reference, is recorded in advance on a part of a recording surface of a magnetic disk. Then, servo signals are sequentially recorded at a predetermined head-traveling pitch, while a magnetic head controls the positioning thereof by reading out the reference signal and detecting the position thereof. Although the self-servo write method has the advantage that no servo track recording device is required, it is pointed out that there are problems in how to record the reference signal, used as the reference for the positioning of the magnetic head, on the magnetic disk with high precision and at low cost. Therefore, in order to solve such problems, a self-servo write technique for recording a reference signal by the above-described magnetic printing method is proposed (see, for example, Japanese Unexamined Patent Publication No. 2001-243733).
However, in any of these methods, although servo signals are sequentially recorded at a predetermined head-traveling pitch, the head-traveling pitch during the recording of the servo signals is determined based on a data track pitch. In order to realize a highly reliable magnetic disk that enables accurate recording/reproduction of data, an optimum or suitable (hereinafter, simply called “optimum”) data track pitch has to be determined, and servo signals need to be recorded at a predetermined head-traveling pitch calculated based on the data track pitch. An optimum data track pitch is determined based on various characteristics such as the gap lengths and characteristics of a reproducing element and a recording element of a magnetic head, the magnetic property of a magnetic disk, and a spacing between the magnetic head and the magnetic disk. Normally, these characteristics are varied for each magnetic head and each magnetic disk. Hereinafter, a method for calculating an optimum data track pitch will be described.
FIG. 6 is a diagram for describing a common method for calculating a data track pitch. The reference number 2 denotes a magnetic head, the reference number 2a denotes a recording element of the magnetic head, and the reference number 2b denotes a reproducing element of the magnetic head. The reference number 2c denotes a deviation (offset amount) of each of the gap centers of the recording element 2a and the reproducing element 2b. It should be noted that the gap lengths of the recording element 2a and the reproducing element 2b are individually different. In calculating a data track pitch, first, a burst signal 20 with a constant frequency is recorded using the recording element 2a. Next, the burst signal 20 is reproduced using the reproducing element 2b. 
If the position of the magnetic head 2 is minutely changed with respect to the position of the magnetic head 2 when the burst signal 20 is recorded, the reproduction output of the burst signal 20 is changed. The relationship between the position of the magnetic head 2 and the reproduction output, which is plotted, is called an “off-track profile”, and is represented by a curve 21. For example, the position of the magnetic head 2 corresponding to a half of the maximum value of the reproduction output is located at positions 21a and 21b, and a distance 22 between these positions 21a and 21b can be set as an optimum data track pitch. Furthermore, by measuring the distance between the position of the magnetic head 2 when the burst signal 20 is recorded, and that of the magnetic head 2 when the reproduction output is maximized, an offset amount 2c can be calculated.
A method for calculating an optimum data track pitch is not limited to the above-described method. As another method for calculating an optimum data track pitch, the following method is known: the relationship between an off-track allowance and a data track pitch is determined with respect to data signals recorded on a plurality of tracks at a predetermined data track pitch, and the data track pitch at which the off-track allowance is maximized is set as an optimum data track pitch (see, for example, R. A. Jensen, J. Mortelmans and R. Hauswizer, “Demonstration of 500 Megabits per Square Inch with Digital Magnetic Recording”, IEEE Trans. Magn., vol. 26, No. 5, pp2169–2171 (1990), and description around column 4, line 22 of USP 06445521). An off-track allowance will be described later.
FIGS. 7A through 7D are diagrams for describing the method. According to the present method, the following steps (1) through (4) are carried out.
(1) As background noise, pre-data signals 30 are recorded on a magnetic disk (see FIG. 7A).
(2) The pre-data signals 30 are overwritten with data signals 31 at a predetermined data track pitch 32 (see FIG. 7B).
(3) The position of a magnetic head (not shown) is minutely changed, and then an off-track characteristic 33 with respect to the error rate of reproduction signals of the data signals 31 is determined (see FIG. 7C). Further, magnetic head positions 34a and 34b when the error rate reaches a predetermined value (BER0) are determined, and a distance 34c between the positions 34a and 34b is determined. This is an off-track allowance.
(4) The data track pitch 32 is changed, and then the steps (2) and (3) are repeated, thus determining the relationship between the data track pitch and the off-track allowance (see FIG. 7D). A curve 35 representing the relationship between the data track pitch and the off-track allowance is generally called a “747 curve”. Based on the 747 curve 35, a data track pitch 32a at which the off-track allowance is maximized is determined, and this data track pitch 32a is set as an optimum data track pitch.
As apparent from the measurement of the 747 curve, there is a close relationship between the data track pitch and the error rate of the reproduction signals, and the error rate of the reproduction signals varies depending on how the data track pitch is set. Therefore, in order to reduce read error in regard to the reproduction signals so that the reliability of a magnetic disk apparatus is improved, it is important to calculate an optimum data track pitch.
As a technique for determining a head-traveling pitch during recording of servo signals by using a self-servo write method, the following technique has already been proposed: a magnetic head is positioned using a reference signal recorded on a recording surface of a magnetic disk, and the above-mentioned off-track profile, for example, is determined to calculate an optimum head-traveling pitch. For example, in the proposed technique, the reference signal, which is used as the reference, is recorded in advance on a part of the recording surface of the magnetic disk, and the reference signal is read out by the magnetic head, thus determining the off-track profile (see, for example, Japanese Patent No. 3251804).
On the other hand, a technique for determining an off-track profile without using a reference signal is also proposed (see, for example, Japanese Unexamined Patent Publication No. 2002-230929). According to this technique, an actuator that supports a magnetic head is pressed against an elastic stopper provided in a magnetic disk apparatus, and an off-track profile is determined from the relationship between electric current for driving the actuator and reproduction output.
In the method for calculating an optimum data track pitch using an off-track profile and/or a 747 curve, for example, it is necessary to position a magnetic head with high precision and then to determine the reproduction output and/or error rate of signals.
In the method for recording servo signals using a dedicated servo track recording device, a magnetic head can be precisely positioned by using an external actuator. Since an off-track profile and/or a 747 curve are/is determined after the precise positioning of the magnetic head, the accurate calculation of an optimum data track pitch is theoretically possible. However, in recording servo signals, in addition to the step of recording servo signals themselves, the step of calculating a data track pitch is required. Therefore, a period of time during which the servo track recording device is exclusively used for a single magnetic disk apparatus is further extended, which might cause a further reduction in productivity. Accordingly, the fact is that at a site where magnetic disk apparatuses are mass-produced, an optimum data track pitch is not calculated for each magnetic disk apparatus, but a specific track pitch, determined averagely by considering variations in sizes and/or characteristics of recording and reproducing elements of the magnetic head, is uniformly set as an optimum data track pitch. Thus, if variations in various characteristics, such as the size and/or characteristic of each element of a magnetic head, the magnetic property of a magnetic disk, and a spacing between the magnetic head and the magnetic disk, are great, the resulting magnetic disk apparatus cannot achieve a desired performance, and the yield is reduced.
On the other hand, in the conventional technique for recording servo signals using a self-servo write method, it is difficult to say that the accuracy in calculating an optimum data track pitch is sufficiently high.
According to the technique disclosed in Japanese Patent No. 3251804, provisional servo information is recorded in advance on a part of a magnetic disk, and servo information is recorded using the provisional servo information. In order to calculate an optimum data track pitch with high accuracy, it is important to increase the accuracy of the provisional servo information that is recorded in advance. Therefore, in the technique disclosed in the above publication, the provisional servo information is recorded using a dedicated external device. However, since the dedicated external device is used, the advantage of the self-servo write method itself is reduced. Furthermore, since the provisional servo information is recorded only in a part of a recording region, the calculation of an optimum data track pitch is effective only for the recording region of the provisional servo information, and a data track pitch different from the optimum data track pitch is undesirably set for the other recording region.
According to the technique disclosed in Japanese Unexamined Patent Publication No. 2002-230929, a reference signal is not used, but an elastic stopper is utilized; however, in calculating a data track pitch, the deformation property of the elastic stopper becomes an error factor, and thus the data track pitch cannot be accurately calculated.
In a normal magnetic disk apparatus, a magnetic head is rotated around a rotation bearing (pivot) by a voice coil motor (VCM), and is moved from the outer radius of a magnetic disk to the inner radius of the magnetic disk (or from the inner radius to the outer radius). That is, the magnetic head can be moved substantially in a radial direction of the magnetic disk. However, since the magnetic head is rotated around the rotation bearing, the longitudinal direction of the head does not necessarily correspond to the radial direction of the magnetic disk, and a skew angle exists depending on the radial position of a recording track. FIGS. 8A and 8B are schematic diagrams illustrating the positional relationship among the recording element 2a, the reproducing element 2b and a recording track 41 in the case where no skew angle exists and in the case where a skew angle exists, respectively. As can be seen from FIGS. 8A and 8B, the projected length of the magnetic head in the radial direction, i.e., a substantial recording element width 2d, varies between the case where no skew angle exists (see FIG. 8A) and the case where a skew angle exists (see FIG. 8B). Besides, the substantial recording element width 2d also varies depending on the magnitude of the skew angle. Therefore, an optimum data track pitch 40 varies for each recording track position.
Thus, in a magnetic disk apparatus, an optimum data track pitch varies depending on the recording track position (radial position) of a magnetic disk. However, any of the above-described methods cannot cope with a variation in the optimum data track pitch due to the track position.